Neutron detectors are useful in many industries. They are most commonly found in the oil industry to detect potential oil yielding sites. Oil producing formations deep in the earth emit neutrons at a different rate than water bearing formations or non-fluid bearing rock. A device using neutron detection for logging oil wells is disclosed in U.S. Pat. No. 4,641,028 to Taylor et al. issued on Feb. 3, 1987. Neutrons detectors are also useful in the medical field and for surveillance in nuclear facilities and weapons storage.
Neutrons are uncharged particles and do not ionize matter as they pass through it. Therefore, they are difficult to detect directly. Some other evidence of a neutron event must be detected in order to determine its existence. An indirect method detects the result of a neutron event and not the neutron event itself.
The use of indirect detection of neutrons is known in the art. For example, a neutron detector as disclosed in U.S. Pat. No. 5,334,840 to Newacheck et al. issued Aug. 2, 1994 detects photons of light emitted by carbon infiltrated boron nitride in its hexagonal form when the compound is bombarded by neutrons. The amount of light detected correlates to the number of neutrons bombarding the boron nitride.
Another neutron detector commercially available utilizes Helium-3 as the neutron absorber. When bombarded by neutrons, Helium-3 decomposes into H and H.sub.3 while emitting electrons with an energy of 764 keV. The ionization of the gas electrons can be detected using conventional methods well known in the art and further described below. This type of neutron detector requires a long collection time for the resulting ionization requiring integrating and differentiating time constants of between 1 and 5 microseconds for the best results.
Other gas mixtures are commercially available that have varying resolution or charge per pulse yields depending on the gases used.
Neutron detection for monitoring the dose of thermal neutrons given patients receiving boron neutron-capture therapy have used lithium-6 and a cerium activator in a glass fiber. See reference 1. The present invention improves upon this technique by allowing a cerium activated glass fiber to be coated with the fissionable elements as described herein. A wet chemistry method of placing radioactive fissile elements into glass which in the vitrified state does not pose a hazard, as described herein using sol-gel based technology, is a significant benefit. Reference 10 describes an avalanche photodiode implementation for detecting neutron induced ionization and optical pulse detection.
The present invention incorporates sol-gel techniques not heretofore used in the art of neutron detection. Sol-gel chemistry was first discovered in the late 1800s. This area of chemistry has received renewed interest when the process was found useful in producing monolithic inorganic gels at low temperatures that could be converted to glasses without a high temperature melting process. A comprehensive explanation of sol-gel chemistry may be found in reference 2. Further detail disclosing uranyl-doped sol-gel glasses is disclosed in reference 3.
Emissions detectors such as microchannel plates, channeltrons, or avalanche photodiodes are in common use for detecting ultraviolet (UV) light and fissioned particles such as electrons. Microchannel plates are commercially available and well known in the art. Typically a microchannel plate is formed from lead glass having a uniform porous structure of millions of tiny holes or microchannels. Each microchannel functions as a channel electron multiplier, relatively independent of adjacent channels. A thin metal electrode is vacuum deposited on both the input and output surfaces to electrically connect channels in parallel. Microchannel plates can be assembled in stacked series to enhance gain and performance.
The microchannel plates serve to amplify emissions from fissionable material resulting from the bombardment of neutrons. The amplified signal which is proportional to the emissions which is in turn proportional to the amount of neutrons bombarding the fissionable material can then be detected and recorded.
Channeltrons operate on the same basic principal of amplifying proportional signals emitted from fissionable materials. A channeltron is a horn-shaped continuous dynode structure that is coated on the inside with an electron emissive material. An ion striking the channeltron creates secondary electrons that have an avalanche effect to create more secondary electrons and finally a current pulse.
Typically due to the exotic materials and sensitivity of the equipment, the neutron detectors currently available are expensive and difficult to maintain. For example Helium-3 is an extremely rare stable isotope and must be separated at considerable expense from the radioactive gas tritium. Furthermore, the use of a gas absorber results in a slower response time than a solid absorber as disclosed herein. The present invention incorporating fissionable material into a sol-gel composition in combination with an emission detector is new to the art and overcomes some the disadvantages of the prior art described herein.